Islamabad, Pakistan (CNN) -- A top Taliban leader has been arrested in the southern Pakistani port city of Karachi, a senior Pakistani military official told CNN.

The official did not say when or how Agha Jan Motasim was detained. He asked not to be identified because he is not authorized to speak to the media.

Motasim had been missing for the past two weeks, according to two Taliban sources. They did not know whether he had been arrested.

Motasim was the finance minister for the Taliban government in Afghanistan before it was overthrown following the U.S.-led invasion in 2001.

His arrest is significant blow to the Taliban, said Muhammad Amir Rana, head of the Pakistan Institute of Peace Studies, a think tank that monitors militant activity. Motasim was very close to Taliban leader Mullah Mohammed Omar, he said.

"He was in his inner court," Rana said.

Motassim is a member of the Taliban's Quetta Shura, a group of senior Afghan Taliban leaders based in Quetta, Pakistan. The group oversees and directs the Taliban's fight against U.S. and international forces in Afghanistan, according to Rana.